Electroplating with tungsten



June 1957 w. D. BUCKINGHAM ELECTROPLATING WITH TUNGSTEN 2 Sheets-Sheet 2Filed May 21, 1954 IN VEN TOR.

W. D. BUCKINGHAM PLATING POTENTIAL VOLTS ACROSS CELL FIG. 2

ATTORNEY United States Patent 6) "ice ELECTROPLATIN G WITH TUNGSTEN-William D. Buckingham, Southampton, N. Y., assignor to The Western UnionTelegraph Company, New York, N. Y., a corporation of New YorkApplication May 21, 1954, Serial-No. 431,374

7 Claims. (Cl. 204-37) This invention relates to a method of platingcoatings of pure, or substantially pure, tungsten, and to an articlehaving a plating of electrodeposited substantially pure tungsten.

Heretofore it has been possible to electrodeposit coatings of tungstenalloys or metals coplated with tungsten, for example, tungsten alloyedwith nickel, iron, tantalum, cobalt and manganese, but so far as I amaware it has not been possible to produce a plating of pure, orsubstantially pure, tungsten by other than vapor deposition methods.Plating eiiected by vapor deposition has not proved entirelysatisfactory; such a plating is not as dense or as adherent or as smoothas desired, and the thermal decomposition method is cumbersome and iswasteful of the plating material.

Various kinds and shapes of articles readily may be plated with themethod of the instant invention, so long as they may be made the cathodein a plating bath. For example, in one type of an electromagnetic highspeed relay a small round' ball of iron or other magnetic materialwithin an evacuated tube is employed as an armature which rolls back andforth alternately to engage two fixed contacts within the tube under theinfluence of the changing magnetic field of the relay in response toreceived telegraph signals. Such a delay is disclosed in my copendingapplication Serial No. 352,355, filed May 1, 1953, and now Patent No.2,732,454. The ball armature should revolve during operation of therelay and should have a surface of a material suitable for electricalcontact purposes. An important characteristic of such a material is thatit will not Weld easily to the material of the fixed contacts. Tungstensatisfies this requirement to the highest degree.

To prevent the ball from sticking due to excessive residual magnetism itmay be annealed to a degree such that it retains sufiicient magnetism tocause rotation but not enough to cause sticking. The step of annealing,however, reduces the hardness of the ball so that after a period of usethe ball generally will show a slight pattern on its surface produced byinnumerable very small fiat areas. To avoid this effect the ball may beplated with a hard.coat ing, and a coating of tungsten offers anexcellent solution, but heretofore no satisfactory method of plating atungsten coating on such ball was available.

The objects of the instant invention are to provide a suitable method ofplating tungsten by electrodeposition and to produce an article having adense, smooth adherent plating of pure, or substantially pure,electrodeposited tungsten.

In the accompanying drawings:

Fig. 1 is a graph depicting the deposition characteristic of aphosphotungstic acid plating bath employed in the method of theinvention; and

Fig. 2 is a graph, drawn on a logarithmic scale, depicting thevolt-ampere characteristic of the plating bath.

The plating bath preferably is prepared at follows: A quantity ofphosphotungstic acid is dissolved in water, the relative quantitiesbeing in the ratio of approximately 80 2,794,775 4 l atented June 4,1957 grams of the. acid to 300 mililiters of water. This ratio may bevaried within reasonable limits, for example, a solution of 20 grams ofthe acid and 100 m1. of water has also been employed, that is, the ratioof acid to water may vary from approximately 1:4 to approximately 1:5.The acid solution is then substantially neutralized by the addition ofan aqueous solution of potassium'hydroxide, the relative quantitiesbeing in the ratio of approximately 20 grams of the hydroxide to 50milliliters of water. Sodium hydroxide or ammonium hydroxide inmolecular equivalents may be used instead of the potassium hydroxide ifdesired.

The hydroxide solution is added, with stirring or other agitation, tothe phosphotungstic acid solution until the pH of the latter solutionequals approximately 7.2 to 7.6. As the hydroxide solution is added, aheavy white precipitate is formed which disappears when the neutralpoint is reached.

The. article to be plated is then immersed in the bath and operates asthe cathode in the forming circuit; a platinum electrode preferably isemployed as the anode. Small articles, such as the ball armature abovementioned, may conveniently be suspended in the bath by means of anickel-plated Alnico magnet. The bath is operated at a temperature offrom 60 C. to C. with the specific gravity or density in the range of1.175 to 1.250.

The voltage across the electrodes of the plating both must be maintainedwithin close limits. For example, with a nickel cathode area of 1 sq.in., a platinum anode area of 3 /2 sq. in., and a bath temperature of 80C. with a density of approximately 1.17 5 and a pH value of 7.2-7.6, thefollowing conditions have been found to prevail:

Current Through Bath in m. a. Thickness of coat in See Volts 5 min.,note 0 1 2 3 4 5 inches Min Min Min Min Min Min Note #1: No visibleaction occurs at this point.

Note #2:.A blue color becomes slightly visible about the cathode. mNogcet1:2 3: A definite black coating forms on the cathode but comes off SDI1 011.

Note #4: A black coatfornis on the cathode with no visible bubbling.

Note #5: A very adherent coat forms.

a Note #6: This coat slightly less adherent, than at 2.45 volts.

Note#7: Gassing at cathode knocks ofi flakes of coating resulting invery rough coat. Gassing becomes increasingly violent as voltageincreases: from this point. The coat is also increasingly more rough andless adherent.

From the foregoing table and the observed results it is shown that thepermissible range of the plating voltage is critical, i. e.,approximately 2.35-2.65 volts; the optimum value lies within the rangeof 2.4-2.6 volts, and preferably is 2.45 volts. Fig. 1 of the drawingsgraphically depicts the best working range of the bath; the ordinatesshow the thickness of the black or bluish-black coating of a complexintermediate oxide or hydrated oxide of tungsten that forms on theobject, and the abscissas show the values of the plating potential forthe best working range. Fig. 2 shows on a logarithmic scale thevolt-ampere characteristic of the bath. It will be understood that thedensity of the current through the plating bath is that which obtainswhen the cathode has an area of 1 sq. in. exposed to the bath. With acathode having a smaller area, for example, a relay ball armature of thecharacter hereinbefore stated, the cathode area I will be less than 1sq. in. and the values of current density in milliamperes will becorrespondingly smaller than those set forth in the table anddepicte'diin the graphsfor the same rate of 'deposit'iof the coating,since current density involves the concept .of area as well as current.The plating voltage, however, isheld within the limits above given byany suitable means, for example, a potentiometer or voltage divideracross the output of a 6-volt storage battery and a galvanometer relayfor controlling a charging circuit for the battery.

After the black or bluish-black oxide coating has formed on the objectto a thickness of the order of a thousandth of an inch, the object isremoved from the plating bath, and is then heated in a furnace in areducing atmosphere, such as hydrogen, endogas, carbon monoxide or thelike, at or above an incandescent temperature, and preferably about 1000C., for approximately one-half hour. There then remains a very thinadherent coating of pure, or substantially pure, tungsten; if thethickness of the oxide coating was approximately 0.001 inch, in thereducing furnace the tungsten coating drops to about 0.0001 or 0.0002inch which is approximately to that of the oxide coating. The object isagain placed in the plating bath and a second coating plated thereonunder the conditions above set forth, which second coating is alsoheated in a reducing atmosphere in the manner stated above. Successivethin coatings thus applied result in a dense and compact finishedcoating of desired thickness. The oxide coating which forms on thenickel plated magnet that holds the ball or other object in the bath iswashed otf after each plating operation. Between successive coating andheating cycles the ball preferably is worked in a raceway under pressureto increase the density, ductility and toughness of the tungsten layer;other objects of different configuration also may be worked by rollingunder pressure or in any suitable manner for the same purpose.

The composition of the complex oxide'coating which is formed on theobject is not determinable by ordinary analytical methods includingspectographic, and it is not known whether it is a single compound or amixture. It has been determined, however, that in the reducing furnacethe coating is reduced to' pure, or substantially pure, tungsten. Forbrevity in the claims the expression an oxide of tungsten is employed todefine this complex compound.

What is claimed is:

l. The method of forming a coating of tungsten on' an object whichcomprises making said object the cathode in a plating bath of an aqueoussolution of phosphotungstic acid to which has been added an aqueoussolution of an alkali hydroxide in an amount to cause the pH of the bathto have a value of approximately 7.2 to 7.6 with a specific gravity inthe range 1.175 to 1.250, fpassing a plating current through said bath,maintaining the temperature of said bath at approximately 60 C. to 80C., controlling the plating voltage to a value of approximately 2.35 to2.65 volts to cause to be deposited on the object a thin adherentcoating of an oxide of tungsten, removing the object from the platingbath and heating the same in a reducing atmosphere at an elevatedtemperature until said coating is reduced to a thin adherent layer ofsubstantially pure tungsten, and repeating the foregoing plating andreducing cycle until a layer of tungsten of desired thickness isdeposited on said object.

2. The method according to claim 1, in which the object is kept in thebath during each plating operation until a coating of said oxide oftungsten of approximately .0006 to .001 of an inch has been deposited onthe object, and in which said coating is heated in the reducingatmosphere until a layer of approximately 71 to A; of that of theoxide-coating remains during each plating and reducing cycle. 7

3. The method according to claim 1, in which the tungsten layer isworked under pressure between successive plating and reducing-cycles toincrease the density and toughness of the coating.

4. The method of forming a coating of tungsten on an object whichcomprises making said object the cathode in a plating bath of an aqueoussolution of phosphotungstic acid in which the ratio of acid to water isapproximately 1:4 to 1:5 and to which has been added a sufficientamountof an aqueous solution of an alkali hydroxide of approximately 40%concentration by weight to cause the pH of the bath to have a value ofapproximately 7.2 to 7.6 with a specific gravity in the range 1.175 to1.250, passing a plating current through said bath, maintaining thetemperature of said bath at approximately C. to C., controlling theplating voltage to a value of approximately 2.35 to 2.65 volts to causeto be deposited on the object a thin adherent coating of an oxide oftungsten, removing the object from the plating bath and heating the samein a reducing atmosphere at an elevated temperature until said coatingis reduced to a thin adherent layer of substantially pure tungsten, andrepeating the foregoing plating and reducing cycle until a layer oftungsten of desired thickness is deposited on said object.

5. The method according to claim 4, in which said alkali hydroxideconsists essentially of a compound of the class consisting of potassiumhydroxide, sodium hydroxide and ammonium hydroxide.

6. The method according to claim 4, in which said basic compound ispotassium hydroxide.

7. The method according to claim 4, in which said plating voltage iscontrolled to a value of substantially 2.45 volts.

OTHER REFERENCES Holt et al., Metal Industry (N. Y.), vol. 31 (1933),pp. 94-97. r

1. THE METHOD OF FORMING A COATING OF TUNGSTEN ON AN OBJECT WHICHCOMPRISES MAKING SAID OBJECT THE CATHODE IN A PLATING BATH OF AN AQUEOUSSOLUTION OF PHOSPHOTUNGSTIC ACID TO WHICH HAS BEEN ADDED AN AQUEOUSSOLUTION OF AN ALKALI HYDROXIDE IN AN AMOUNT TO CAUSE THE PH OF THE BATHTO HAVE A VALUE OF APPROXIMATELY 7.2 TO 7.6 WITH A SPECIFIC GRAVITY INTHE RANGE 1.175 TO 1.250, PASSING A PLATING CURRENT THROUGH SAID BATH,MAINTAINING THE TEMPERATURE OF SAID BATH AT APPROXIMATELY 60*C. TO80*C., CONTROLLING THE PLATING VOLTAGE TO A VALUE OF APPROXIMATELY 2.35TO 2.65 VOLTS TO CAUSE TO BE DEPOSITED ON THE OBJECT A THIN ADHERENTCOATING OF AN OXIDE OF TUNGSTEN, REMOVING THE OBJECT FROM THE PLATINGBATH AND HEATING THE SAME IN A REDUCING ATMOSPHERE AT AN ELEVATEDTEMPERATURE UNTIL SAID COATING IS REDUCED TO A THIN ADHERENT LAYER OFSUBSTANTIALLY PURE TUNGSTEN, AND REPEATING THE FOREGOING PLATING ANDREDUCING CYCLE UNTIL A LAYER OF TUNGSTEN OF DESIRED THICKNESS ISDEPOSITED ON SAID OBJECT.